Rio 2016 Olympics: Five things to watch on Day 1

(CNN) -- The spectacular opening ceremony is over, the stadium flame has been lit, and after months of uncertainty, attention turns to the main event -- the sport at Rio 2016.

Twelve gold medals will be awarded Saturday and 21 sports will be contested as the action gets underway.

The first podium places of the Games will be decided in the women's 10-meter air rifle competition, but after the Russian team escaped a blanket ban following the doping scandal could the first medal of Rio 2016 go to a Russian competitor -- Daria Vdovina?

There's plenty of action on the road, in the water and in the ring as stars from cycling, rowing, gymnastics, boxing and basketball all showcase their skills.

Here are five highlights from the first full day of sporting endeavor:

Cycling

After enduring the grueling three-week Tour de France, many of the road race cyclists now face a difficult and steep course around Rio. This year's Tour winner Chris Froome is supported by a strong British team who will have to tackle the course of 237 kilometers (147 miles) which includes tough climbs, a tricky descent and a finish at Copacabana beach.

The hilly nature of the route will make it hard -- though not impossible -- for the sprint specialists so the medal places could go to the strongest climbers. The field will be relieved to see that the world champion, Slovakian Peter Sagan, has opted out of the road race and will compete in the mountain bike competition instead.

Some of the favorites for gold include Julian Alaphilippe (France), Vincenzo Nibali (Italy), Alejandro Valverde (Spain) and Dutch cyclists Tom Dumoulin, Wout Poels and Bauke Mollema. But watch out for a surprise. There is always a chance that another courageous rider could escape the pack, risk all on the technical sections of the descent and race home to Olympic glory on the beach.

Swimming

Three gold medals will be won in the pool. Both the men's and women's 400-meter individual medley will be decided Saturday along with the men's 400m freestyle competition. The 2012 Olympic champion, China's Sun Yang, will be defending his title in the freestyle but will face stiff opposition from Mack Horton (Australia) and American Connor Jaeger.

In the women's medley, Hungarian Katinka Hosszu will start as favorite but will be challenged by Britain's Hannah Miley, Spain's Mireia Belmonte and American Elizabeth Beisel.

Heats also start in the women's 100m butterfly, men's 100m breaststroke and the women's 4x100m team relay race.

Rowing

American rowers will compete in their antimicrobial suits after fears about water quality at the Games.

Last year, scores of dead fish appeared in the Rodrigo de Freitas lagoon, where the Olympic rowing and canoe sprint events will take place.

Heats for the men's and women's single sculls and pairs begin Saturday -- and among them will be the sole Indian rower Dattu Bhokanal.

Bhokanal has certainly qualified the hard way. He lives in a one-room mud hut in Talegaon Rohi village, in India's Maharashtra state, with no running water, no television and no refrigerator.

He grew up in a drought-prone region where monsoon rains were erratic, and only started rowing when he joined the army in 2011.

He says a medal is a long shot but perhaps Saturday could perhaps be his day of glory.

Basketball

The U.S. has dominated Olympic basketball in both the men's and women's competitions, and both teams again start as favorites.

The men have only been beaten to gold four times since 1936 -- the last time in 2004 when Argentina took top spot -- and the women have won gold in the last five Olympic Games.

First-round matches begin Saturday with the U.S. men's team facing China. It's a chance for the rest of the world to marvel at leading basketball stars like Kevin Durant and Draymond Green.

Rugby returns to the Olympic schedule for the first time since 1924 -- this time in the seven-a-side version of the game -- and the host nation is in action in the first pool matches with the women's team taking on the British.

Rugby sevens is a game of constant, furious activity and it is exhausting just to watch. The seven players have to marshal a full-size pitch, so play is restricted to just seven minutes per half.

Both the men's and women's competitions feature 12 teams, with the latter opening proceedings Saturday in the temporary Deodoro Stadium -- which will also host parts of the pentathlon event. The men's tournament begins Tuesday.

The team to watch in the men's battle will be Fiji -- winner of the past two World Series titles -- while Australia is women's favorite after winning its 2015-16 World Series.